Modification of the Al-Si eutectic microstructure has been studied by using the entrained droplet technique in a range of hypoeutectic Al-Si alloys doped with different levels of phosphorus and sodium. Differential scanning calorimetry has been used to investigate the kinetics of silicon nucleation during eutectic solidification, and transmission and scanning transmission electron microscopy has been used to investigate the eutectic microstructure and microchemistry. The Al-Si eutectic microstructure can be modified from coarse-faceted silicon particles nucleating at low undercooling to clusters of fine-scale silicon particles nucleating at high undercooling by either increasing alloy purity, adding sodium, or increasing cooling rate during solidification. The unmodified coarse silicon particles are heterogeneously nucleated on pre-existing AlP particles. Increasing alloy purity removes phosphorus and prevents the formation of AlP; adding sodium leads to the formation of Na3P in preference to AlP; and increasing cooling rate allows insufficient time for the precipitation of AlP from low levels of dissolved phosphorus. At a cooling rate of similar to 10 K min(-1), the transition from an unmodified to a modified Al-Si eutectic microstructure takes place when phosphorus levels fall below 0.25-2 p.p.m., and when sodium levels increase to 80-850 p.p.m.